Filtering screen and support frame therefor

ABSTRACT

A method of constructing a frame ( 10 ) for a filtering screen from a polymer material is described in which a wire frame reinforcement is wholly encapsulated during the frame moulding process so that two parallel spaced apart wires extend through each of the intersecting orthogonal ribs ( 14, 16 ) which define the open area of the frame ( 10 ) across which the wire-cloths ( 158, 159 ) are stretched. The surfaces of the frame ( 10 ) in which the wirecloths ( 158, 159 ) are embedded are formed with ridges ( 22, 30, 32 ) and the crests of the latter extend to different heights. The crest are softened by heating during the manufacturing process and the wirecloths ( 158, 159 ) stretched over the frame ( 10 ) are forced into the softened crests after which the assembly is allowed to cool and cure. The frame ( 10 ) is re-usable by stripping wirecloths ( 158, 159 ) from the top of the frame ( 10 ) and fitting fresh wirecloths over the plastics material and heating the regions which are to encapsulate the fresh wirecloth.

FIELD OF INVENTION

[0001] This invention concerns screens for use as filters in vibratoryfiltration equipment such as shakers that are used in the oil drillingindustry for separating solids from the liquid phase of oil and waterbased muds retrieved from drilling operations. In particular theinvention is concerned with the construction and manufacture ofsupporting frames over which filtering material such as woven wire clothis stretched and secured to form the screens and to the re-use of suchframes.

BACKGROUND

[0002] Examples of filtering machines in which such screens are used arecontained in UK 2237521 and UK 2229771.

[0003] One such screen is described in PCT/GB95/00411 (W095/23655).

[0004] The earlier design of screen extends the life of a screen byproviding a sacrificial support cloth of woven wire below an upper wovenwire cloth of harder wearing material than that of at least the surfaceof the wire from which the lower cloth is woven, so that wear due torubbing and vibration during use occurs to a greater extent in the lowercloths than in the upper cloth. The specification also describes animproved design of frame across which woven wire cloths can be tensionedand bonded by adhesive, to form a sifting screen, in which the frame isproposed to be formed from glass reinforced gas blown polypropylene withelongate metal reinforcing elements or rods buried in the GRP. Theimproved frame construction is shown in FIGS. 3 to 8 of the earlierspecification.

[0005] It is an object of the present invention to provide an improvedmethod of constructing such a frame and an improved frame for use as thesupport for layers of woven wire cloth, an improved screen formed-fromsuch a frame, and a frame which can be reused under some circumstances.

SUMMARY OF THE INVENTION

[0006] According to one aspect of the invention, a method ofconstructing a polymer support frame over which woven wire cloth is tobe stretched and secured to form a sifting screen comprises the steps oflocating in a mould tool a wire frame assembly comprising two parallelspaced apart arrays of reinforcing wires, closing the tool, injectingliquid polymer so as to wholly encapsulate the wire frame and to form anarticle having an open central region crisscrossed by intersectingorthogonal ribs bounded on all sides by a rigid flange, in which each ofthe ribs includes two parallel spaced apart wires of the said wire frameassembly, permitting the polymer to cure, and opening he tool, andremoving the moulded article.

[0007] By arranging for two parallel spaced apart wires to extendthrough each of the ribs, one near one edge and the other neaer to theopposite edge of the rigs, each rib has the stiffness of a beam, and theresulting frame has high rigidity and resistance to bending, yet remainsrelatively lightweight.

[0008] Preferably the wire frame is selected so as to impart sufficientstructural rigidity to the support frame as to prevent deflectionthereof and consequent changes in the tension in the wire cloth whenfitted thereto.

[0009] Additionally the wire frame is selected so as to impartsufficient strength to the support frame as to allow the latter towithstand shear stresses introduced as the frame is clamped into avibratory screening machine.

[0010] Typically the wire frame is formed from high tensile straightenedsteel wire, bent as required, and in a preferred arrangement the wire isof 2.5 mm diameter.

[0011] The invention also provides a method of making a reinforcing wireframe for use in the above method of making a support frame, comprisingthe steps of:

[0012] equally spacing apart cut lengths of wire in a jig to form afirst array,

[0013] locating thereover a second equally spaced array of cut lengthsof wire at right angles to the first array,

[0014] resistance welding the wires of the two arrays at all the pointsof intersection so as to form a first rectilinear matrix

[0015] similary positioning two similar arrays of similarly cut lengthsof wire in a jig and resistance welding the points of intersection ofthe orthogonal wires so as to form a second, similar rectilinear matrix,

[0016] bending in a press break the protruding lengths of wire on atleast two of the four sides of one of the rectilinear matrices, so as tobend each protruding section first in a generally upward sense and thenat a point nearer to its end in a downward sense so that the end regionof each protruding length extends parallel to the plane containing thematrix, but is displaced therefrom, and

[0017] and thereafter resistance welding the displaced ends of theprotruding wires of the said one matrix to the protruding ends of thewires of the other matrix.

[0018] The welding may be in part effected through the intermediary oftransversely extending filler wires, so that intersections are providedwhere welds are to be formed between parallel protruding ends of thereinforcing wires, and the filler wires facilitate the resistancewelding of the parallel protruding ends.

[0019] Preferably the wire frame fabrication is assembled so that eachmatrix is bowed in an outward sense, opposite to the other.

[0020] During moulding it has been found that the pressure within themould tool can distort the framework so that the outward bowing of theopposite faces of the framework can be replaced by significant inwardbending—so destroying the alignment of the long rods within the upperand lower edges of the interstices of the moulded frame. To avoid thisit is proposed that at least one spacer is located within the framework,so that if there is any tendency for the rod arrays to collapse inwards,the spacer will present this collapse occurring.

[0021] Preferably a plurality of spacers are located within the wireframe fabrication, each attached to one or other of the matrices so asto extend towards the other, whereby any tendency for the matrices tocollapse inwards during moulding, is resisted by the spacers.

[0022] In one embodiment each spacer comprises a length of wire bent toform a shallow U with its two ends bent outwards to form two in-linelugs by which it can be welded to the underside of one of the wireswhich form one of the matrices, with the crest of the U section in closeproximity to one of the wires of the other matrix, whereby the spacerwill maintain a given dimension between the two matrices if thefabrication is subjected to a collapsing force during moulding, socausing the crest to engage the said wire of the other matrix.

[0023] The invention also provides a method of moulding a support framein a mould tool as aforesaid around a wire frame fabrication asaforesaid wherein an inward force is exerted on opposite faces of thefabrication within the mould tool by fingers protruding inwardly fromthe inside faces of the tool, to externally engage the opposite matricesof the fabrication when the tooling closes.

[0024] In accordance with this method, the fingers sandwich thefabrication in position and produce just the required inward movement ofthe two oppositely bowed matrices to render them parallel and spacedapart by the desired distance.

[0025] Typically the fingers comprise inwardly projecting pegs whichalign with crossing points of wires in the upper and lower reinforcingmatrices, to space the matrices from the corresponding upper and lowerinternal surfaces of the mould tool and ensure that the matrices areburied within the plastics material which is injected into the mouldtool during the manufacturing process.

[0026] Preferably the ends of the peas taper to an edge, or a point.

[0027] After the mould tool is opened and the protruding pegs disengagefrom the struts, openings are left in the polymer. Preferably thereforethe method further comprises plugging the openings with plasticsmaterial or filler.

[0028] Typically the wire frame fabrication. is supported within thetooling by means of retractable pins which protrude through the toolingwall to engage the fabrication and accurately locate it within thetooling.

[0029] The pins may be retracted as the tooling opens after the mouldingstep has been completed.

[0030] Conveniently the pins align with protruding ends of wires makingup the fabrication and are separably joined to the ends of the wires bymeans of sleeves of plastics material opposite ends of which receive thepins and the reinforcing wire ends respectively.

[0031] Preferably the passage through each sleeve is blocked so as toform two coaxial blind bores, and each sleeve becomes embedded in thepolymer during moulding and remains in the polymer as the pin whichengages it is retracted as the tooling is opened, the blocked passageserving to encapsulate the end of the wire end located in the inner endof the sleeve.

[0032] Prior to moulding the tool may be fitted with pegs formed from aplastics material which is compatible with or is the same as the polymermaterial which is to be injected into the mould to encapsulate the wireframe fabrication, and the pegs become integrally bonded therein duringmoulding so that when the tool is opened, the pegs separate from thetool, and remain in the frame.

[0033] Protruding portions of each peg may be removed by grinding orfiling or cutting.

[0034] The invention thus provides a support frame of polymer materialhaving wire reinforcing thereon and to which woven wirecloth is to bebonded to form a filtering screen, in which the frame is constructed asaforesaid.

[0035] The invention also provides a filtering screen comprising asupport frame of polymer material with wire reinforcing thereinconstructed as aforesaid, and having at least one woven wireclothtensioned thereover and bonded to one surface thereof.

[0036] It is generally desirable for the wires of the cloths to be tautand under tension in the finished product and to this end the wireclothswires are put in tension whilst the plastics material is allowed to cooland set hard, to bond the woven wire cloths to the frame.

[0037] This aspect of the invention is so particular advantage in thatby using a suitable polymer for the frame material and heating thesurfaces in contact with :he wire cloths, so the need for an adhesive isobviated as is the step of applying the adhesive.

[0038] It is of course necessary to select a plastics material which issuitable to serve as a bonding medium for woven wire cloth and it hasbeen found that polypropylene and polyethylene are suitable plasticsmaterials although the invention is not limited to the use ofpolypropylene and polyethylene.

[0039] Whether polyethylene or polypropylene is used, it has beer. foundadvantageous for either material to be gas blown and glass fibrereinforced.

[0040] In a preferred arrangement, the wirecloth which is first fittedover the support frame has a coarser mesh than any subsequent layer ofwirecloth fitted thereover.

[0041] In particular the invention provides a filtering screen havingtwo layers cc woven wirecloth flitted thereover and bonded thereto, inwhich the lower wirecloth has a coarser mesh than the upper wireclothand in which the tension in the wires forming the upper wirecloth isless than the tension in the wires forming the lower wirecloth.

[0042] According to another aspect of the invention, a wire framereinforcing fabrication for a support frame as aforesaid, is formed fromresistance welded steel wire matrices arranged in two parallel spacedapart planes and themselves welded together along at least two edgeregions by welds between protruding ends of the wires of the twomatrices.

[0043] According to another aspect of the invention, in a method offorming a wire frame reinforcing cage for incorporating in a mould toolfor moulding a polymer material therearound to form a support frame fora filter screen, wherein the cage is constructed from two similarrectilinear arrays of resistance welded wires, the method involvesbending protruding wires along at least two sides of one of the arraysand welding the ends of the bent portions of the wires of the one arrayto the protruding ends of the wires in the other array, so as tomaintain separation between the two matrices.

[0044] According to a further aspect of the invention, a method ofconstructing a filter screen comprises the steps of forming a polymersupport frame having wherein a plurality of similarly sized rectilinearapertures defined by an integral rectilinear matrix of wire reinforcedstruts of polymer material, in which the upper edge of each strut, andthe upper surface of each boundary of the support frame, is ridged, andin which the woven wirecloths are fitted over the ridges, tensioned, andsecured in place by heating at least the ridges so as to soften thepolymer material therein sufficiently to allow the wirecloths topenetrate the crests thereof and upon cooling to remain embeddedtherein, so as to maintain tension in the wires of the wirecloths aftercooling.

[0045] The method may also involve the step of differentially tensioningthe wires in one wirecloth relative to those in the other, so thatdifferent tensions exist in the wares of the two cloths after bonding tothe polymer support frame.

[0046] The reinforcing matrix is preferably formed from high tensilesteel wire of nominally

[0047]2.5 mm diameter, and glass reinforced gas blown polypropylenepolymer is injected into the mould tool under pressure and is left tocure for a given period of time, and the moulded support frame thenremoved, a first woven wirecloth of nominally 30 mesh formed fromstainless steel wire of nominally 0.28 mm diameter is fitted over thesupport frame and tensioned, a second woven wirecloth is fitted over thefirst wirecloth, the second cloth being formed from wire having asmaller diameter and a finer mesh size than the first cloth, and islikewise tensioned, a force is applied over the face of the supportframe carrying the overlaid wirecloths, heat is applied to soften thecrests of the ridges and to allow the two cloths to become embedded inthe crests of the ridges such that after the heating and force isremoved and the frame has cooled, the cloths remain bonded to the crestsof the ridges and residual tensions exist in the wires forming the twocloths.

[0048] In a support frame which is to have woven wirecloth bonded to theone face thereof by locally heating the frame material and forcing thewirecloth wires into the softened material before it is allowed to cooland harden again, and which is formed from glass reinforced plasticsmaterial having embedded therein a reinforcing wire frame constructed asaforesaid, preferably the thickness of the plastics material between thereinforcing wires and the face of the support frame members to which thewoven wirecloth is to be bonded is selected so as to be sufficient toenable the wirecloth to be embedded therein without making contact withthe reinforcing wires.

[0049] Preferably the plastics material comprises a polypropylene or apolyethylene, and may be gas blown and glass fibre reinforced.

[0050] According to a preferred feature of the invention, in a supportframe which is formed from glass reinforced plastics material, the facesof the peripheral edge regions of the support frame which wovenwirecloth is to overlay and to which the cloth is to be bonded byheating, are formed with a plurality of closely spaced apart parallelridges so that when the surface is heated, the crests of the ridgessoften, and woven wirecloth laid thereover and tensioned can, under anappropriate downward loading, penetrate and become embedded in thesoftened crests.

[0051] In a method or constructing a filter screen using a support frameas aforesaid the heat is preferably applied through the woven wireclothso as to preferentially heat the crests of the ridges as opposed to theremainder of the support frame.

[0052] In a support frame as aforesaid, which is generally rectangular,the ridges along each of the four sides run parallel to the lengthdimension of each side so that the ridges in the surfaces of the fouredges of the frame run perpendicular to the direction in which wovenwirecloth is tensioned relative to those edges.

[0053] Where the frame includes a matrix of wire reinforced strutsdefining within the boundary of the frame a plurality of openings,sufficient plastics material exists in each of the struts between theinternal wire reinforcement therein and the cloth engaging surfacethereof, to enable the strands of wire forming the wirecloth also tobecome embedded in the struts when the frame is heated and anappropriate force is applied, without the wirecloth contacting theinternal reinforcing wires, to prevent electolytic action betweenwirecloth and reinforcing wires where different metals are used.

[0054] According to another preferred feature of the invention, in asupport frame as aforesaid, constructed from glass fibre reinforcedplastics material over which wirecloth is to be stretched and bondedthereto to form a sifting screen and which includes a matrix of strutswithin a perimeter flange of the frame, the thickness of the clothengaging ends of the struts is less than that of the more remote regionsof the struts.

[0055] Preferably the wirecloth engaging edge of each strut is ofreduced section, and on heating the wirecloth wires become embedded inthe said reduced strut sections.

[0056] The reduced strut section is preferably created by forming aridge along the wire receiving edge thereof.

[0057] The invention thus also provides a support frame formed fromglass fibre reinforced plastics material over which wirecloth is to bestretched for bonding thereto to form a sifting screen, and whichincludes an outer peripheral flange surrounding a central regionoccupied by an integral matrix of interconnecting struts wherein theflange and struts are ridged where they are to engage the wirecloth andthe ridges extend to different heights so that the crests lie indifferent parallel planes.

[0058] Preferably the crests of the ridges of the struts defining theintegral matrix lie in a first plane, which is below a second planecontaining the crests of the ridges on the surrounding flange.

[0059] In a method of covering a support frame as aforesaid, heat ispreferably applied uniformly over the entire face of the support frameby means of by a flat heated platen which forces the wirecloth into thecrests of the ridges and the wirecloth has to be pressed into the crestsof the ridges on the surrounding flange before it can enter the crestsor the ridges on the struts making up the integral matrix.

[0060] In such a support frame, outer ridges on some or all of thesurrounding flange may extend to a greater height than inner ridges onthe flange, so that the crests of the outer ridges occupy a third planeabove the second plane, whereby there is a greater volume of polymer tobe melted and spread by the application of heat and pressure to thewirecloth near the outer edges of the peripheral flange, than is thecase near the inner regions of the flange.

[0061] By forming ridges along the wire cloth engaging edges of thestruts, so the area of the woven wire cloth which is actually blinded bybeing embedded in plastics material is reduced to the minimum necessaryto achieve support for the cloths. Thus although the struts may have alateral width considerably greater than the width of their crests, thearea of the woven wire cloth which is available for filtering liquid isonly reduced by the narrow width of the melted crests and is not reducedby the total area of the struts, which would be the case if the width ofthe cloth engaging face of the struts is the same width as the strutsthemselves.

[0062] The struts have so far been described as being ridged and ingeneral they will have a symmetrical cross-section in which the crest ismid-way between two parallel sides of the strut, and the ridge can bethought of as an isosceles triangulation at the upper end of arectangular cross-section of the remainder of the strut. However thisconfiguration is merely a preferred form, and the triangularcross-section of the upper region of the strut may be offset so that thecrest of the ridge is displaced towards one side or the other of thestrut when viewed end on and if the triangle is a right angled trianglethe crest will be aligned with one side or the other of the rectangularsection of the strut, the hypotenuse of the triangle forming a slopingsurface between the crest of the ridge and the other edge of therectangular strut region.

[0063] In general the primary purpose of the ridging of the struts is toreduce the width of the plastics material which is to engage the wovenwire cloth, and the ridging effect can be obtained simply by reducingthe width of the strut lust below its face which is to be bonded to thewere cloth. This width reduction may be achieved by means of a step, soas to define a shoulder, (or two steps on opposite sides to form twoshoulders), and the reduced width region above the shoulder(s) providesthe reduced width plastics face for engaging the woven wire cloth. Thedifference between such an arrangement and a triangular ridge is that inthe former case, the underside of the woven wire cloth immediately tothe left or right of a bonded crest will be completely unobstructed atleast down to the shoulder(s) whereas in the case of a triangular ridge,a sloping surface will be presented to liquid filtering through the wirecloth. In practice however, the stronger triangular section is preferredparticularly since differential softening will occur down the height ofthe ridge upon the application of heat to the crests.

[0064] Preferably the overall shape of the frame is a rectangle, and theflange which extends along the two longer sides is wider than the flangealong the two shorter sides, and at least one of the shorter sides isadapted to be joined to the corresponding shorter side of a similarscreen support frame, constructed in the same way as the first.

[0065] The invention also provides a sifting screen constructed from asupport frame as aforesaid, with wirecloth stretched thereon andhermetically bonded thereto.

[0066] The invention also lies in two screens constructed as aforesaidwhen mounted in a supporting framework within a vibratory screeningmachine, with the shorter edges of the screen support frames joined sothat the two screens form a larger area for filtering.

[0067] The invention also lies in a vibratory screening machine whenfitted with two such screens, wherein one of the screens is mounted soas to be substantially horizontal in use and the other is mounteddownstream of the first and is inclined upwardly in the direction ofmovement of particulate material through the machine.

[0068] The invention also lies in a support frame as aforesaid, whereinone of the shorter ends of the frame is adapted to interlock andsealingly engage with a corresponding edge of an adjoining similarsupport frame such that if the two frames are slid the one towards theother, engagement and sealing occurs merely by the sliding movement ofthe one frame relative to the other.

[0069] The sealing engagement of one frame to the other can be arrangedto occur when the two frames are in line, or when the two frames aremutually angled.

[0070] It is an advantage to form screens in this way and to mount anumber of such screens in a supporting framework within a vibratoryscreening machine with edges abutting so as to form a larger area forfiltering, for a number of reasons.

[0071] Firstly, as already indicated, some designs of shaker require afirst filtering area to be substantially horizontal and a second area tobe inclined upwardly in the direction of particulate movements over theoverall screen. This can be achieved by forming the overall screen fromtwo smaller screens, each constructed as aforesaid and adapted to bejoined edge to edge where the change in inclination is required tooccur.

[0072] Secondly, each individual screen can, be of such a size andweight as can be readily handled by one man.

[0073] Thirdly in the event of damage occurring in one such screen, itcan be simply removed and a fresh screen inserted (sometimes withoutneeding to disturb the undamaged screen(s)).

[0074] In a support frame as aforesaid, preferably the cross section ofeach strut is generally rectangular and the longer dimension isgenerally perpendicular to the plane of the frame, and a firstreinforcing wire extends through each strut near the edge thereof whichis to be bonded to wirecloth, and a second parallel reinforcing wireextends through each strut near to the opposite edge thereof, and thewires are bonded to the plastics material by the moulding step and formtherewith a beam, thereby to impart rigidity to the structure.

[0075] Furthermore, in a support frame as aforesaid, preferably thestruts intersect similar struts which extend at right angles and asecond assembly of parallel reinforcing wires is provided, runningperpendicular to the first assembly of wires in planes proximate thosecontaining the first said assembly, so that a pair of parallel spacedapart wires extends through each of the struts.

[0076] In such a support frame each reinforcing wire which aligns withthe peripheral flange of the frame preferably extends into the flange ateach end thereof, thereby to increase the rigidity of the flange.

[0077] Preferably the ends of the other wire of each pair are bent so asalso to become aligned with the flange of the frame, and the bent endsthereof extend into the said flanges close to the ends of the firstmentioned wire to further assist in reinforcing the said flanges.

[0078] Typically the wires touch at all points of intersection and arepreferably welded at all such, points.

[0079] Preferably the ends of each pair of wires are welded where theyoccupy the flange.

[0080] In a wireframe assembly for a support frame as aforesaid,cross-point engagement may be introduced between parallel wires in theflange by incorporating intermediate transversely extending filler wire,or weld wire, between the wire ends.

[0081] Further reinforcing may be provided in the flange by means ofadditional reinforcing wires extending parallel to the length directionof the flange so as to overlie or underlie the protruding reinforcingwires entering the flange from the struts, and the additionalreinforcing wires are welded to the protruding, reinforcing wire ends.

[0082] The invention leads to a filter screen comprising a support frameas aforesaid, having wirecloth bonded thereto, wherein the wires of thecloths are taut and under tension in the finished product, and to thisend the wirecloths are put in tension at least while the frame materialcools and sets hard, to bond the wirecloths to the frame.

[0083] According to another aspect of the invention, there is provided amethod of repairing or refurbishing a filter screen constructed asaforesaid, wherein worn or damaged wirecloth is stripped from thesurface of the polymer frame, fresh cloth is placed over the frame andtensioned as appropriate and heat is applied so as to soften thesurfaces of the frame over which wirecloth is stretched, so that thelatter can penetrate the softened plastics material and become embeddedtherein, after which the assembly is allowed to cool, the tensioningforce is removed, and the wirecloth edges are trimmed back to thesurrounding flange of the frame.

[0084] In a method of repair or refurbishment as aforesaid plasticsmaterial may be applied to the surface of the frame which is to receivethe wirecloth before the latter is fitted thereover, to provideadditional plastics material for bonding the wirecloth to the frame.

[0085] The stripped support frame may be inserted in a mould and freshpolymer material infected into the mould so as to reform on the surfacesof the frame ridges similar to those which existed when the frame wasfirst manufactured, before the wirecloth is applied thereto.

[0086] The invention thus also provides a repaired or refurbished filterscreen wherein worn or damaged wirecloth has been replaced by freshwirecloth in accordance with the above.

[0087] Apparatus for repairing or refurbishing a filter screen fromwhich worn wirecloth has been stripped conveniently comprises a trayinto which the stripped frame is inserted, wirecloth stretching meanssurrounding the tray including attachment means for securing to theedges of a sheet of wirecloth laid over the upper surface of the framein the tray, tensioning means for exerting tension or the wirecloth. inat least two different directions whilst it is so stretched over theframe, means for heating the frame to soften the uppermost edges of thematrix of struts and periphery of the frame, and means for forcing thewirecloth into the softened material such that after cooling, thewirecloth remains bonded to the polymer frame, and can be trimmed backto the edges of the frame.

[0088] Apparatus as aforesaid may also comprise further attachment meansand tensioning means to enable at least a second sheet of wirecloth tohe secured over the first for bonding to the frame, preferably at thesame time as the first.

[0089] In such apparatus additional means may be provided for applyingplastics material along each of the surfaces to which the wirecloth isto be attached prior to or after wirecloth has been stretched over thesupport frame, so as to increase the amount of plastics materialavailable to bond the wirecloth to the frame.

[0090] The invention also provides a method of repair or refurbishment,wherein after removing worn or damaged wirecloth from a support frame aplastics sheet similar in size and pattern of openings to the supportframe when viewed in plan is placed over the frame which is to berepaired, in alignment therewith, before or after new wirecloth isstretched thereover and before heat and pressure is applied, to provideadditional plastics material to bond the new wirecloth to the frame.

[0091] Filtering machines of the type into which screens as aforesaidcan be fitted, are described in UK Patent Specifications Nos. 223752;and 2299771, but these are intended as examples only and the inventionis not limited to the use of these screens in such machines.

[0092] The invention will now be described by way of example, withreference to the accompanying drawings, in which:

[0093]FIG. 1 is a plan view of a frame over which wire cloths are to bestretched and secured to form a screen for use a filter in a vibratoryfiltration equipment such as a shaker for separating solids from theliquid phase of oil and water based muds;

[0094]FIG. 2 is a cross-sectional view at CC of FIG. 1;

[0095]FIG. 3 is a part section at BB in FIG. 1;

[0096]FIG. 4 is a part section on AA in FIG. 1;

[0097]FIG. 5 is a part end view at D of the frame of FIG. 1 to anenlarged scale;

[0098]FIG. 6 is a Dart end view taken in the direction of arrow D ofFIG. 1 to an enlarged scale;

[0099]FIG. 6 is a plan view to a reduced scale showing two screens, eachconstructed using a frame such as shown in FIG. 1, in aligned edgeabutting relation, to form a larger screening area;

[0100]FIG. 7 is a side elevation in the direction of arrow VII of FIG.6;

[0101] FIGS. 8 or 9 show to an enlarged scale the two inter-engagingedge regions of the two frames of FIGS. 6 and 7;

[0102]FIG. 10 shows to enlarged scale in cross-section the abuttingregion of the two frames shown circled at VIII in FIG. 7;

[0103]FIG. 11 is a plan view from above of another frame similar to thatof FIG. 1, but adapted to be joined in edge abutting relation withanother similar frame so that the two frames are angled relative to oneanother when viewed from the side;

[0104]FIG. 12 is an end view at “B” in FIG. 1;

[0105]FIG. 13 is a section on DD in FIG. 11;

[0106]FIG. 14 is a part end view (to an enlarged scale) at “B” in FIG.11;

[0107]FIG. 15 is a part section a AA of the left hand end of the frameshown in FIG. 11;

[0108]FIG. 16 is a part section at AA of the right hand end of the frameof FIG. 11 showing in more detail the wire reinforcing structure, andwhere inserts are required to support it within a mould tool;

[0109]FIG. 17 is a part section at CC (to an enlarged scale) of theframe of FIG. 11;

[0110]FIGS. 17A and 17B show how the wire frame can be supported withina mould tool by ferrules which become an integral part of the mouldedframe to cover the ends of the wires;

[0111]FIG. 17C shows a bent-wire spacer to prevent wireframe collapse inthe mould tool, as polymer is injected finder pressure.

[0112]FIG. 18 is a side edge view of two frames of the type shown inFIG. 11, in edge abutting engagement;

[0113]FIG. 18A is a view of the join to an enlarged scale;

[0114]FIG. 19 is a side elevation partly in cross-section of anintersection between row and column support members of a frame such asshown in FIG. 1;

[0115]FIG. 20 is a top plan view of the frame part shown in FIG. 19;

[0116]FIG. 21 is an elevation of a support for locating the wirereinforcing structure at a point of intersection as shown in FIGS. 19and 20;

[0117]FIG. 22 is an end view to an enlarged scale of the lower end ofthe support of FIG. 21;

[0118]FIG. 23 shows to a larger scale the preferred engagement of thechisel end of the stand off support with a reinforced rod; and

[0119]FIG. 24 shows part of a completed screen.

DETAILED DESCRIPTION OF THE DRAWINGS

[0120]FIG. 1 illustrates a frame across which woven wire meshes are tobe stretched and bonded in the manner herein described. The frame isgenerally designated 10 and includes a large number of similarly sizedrectangular openings, one of which is denoted by 12 formed by arectilinear matrix of orthogonally intersecting cross-members one ofwhich is designated by 14 and another of which is designated byreference numeral 16.

[0121] On two opposite sides, namely 18 and 20, the frame includes arelatively wide flange the upper surface of which is corrugated so as toform ridges and furrows 22. The other two edges 24, 26 and 28 arerelatively narrow and include just two or three ridges 30, 32. Byforming at least the ridges from a plastics material which can besoftened on heating and which sets hard on cooling, woven wire meshes(not shown) can be stretched over the ridged peripheral regions of theframe and embedded in the ridges by heating the peripheral edges of theframe and pressing the wire meshes into the softened crests of theridges. By allowing the plastics material to cool, so the wire mesheswill be held firmly in place and if they are tensioned in bothdirections prior to the heating step and the tensions are maintainedthroughout, thereafter the plastics material has cooled, residualtensions will remain in the stretched layers of wire cloth. If as isusually preferred, the wire cloths have been differentially tensioned sothat the tension in one is higher than that in the other, thedifferential will remain in the residual tensions retained in the wirecloths.

[0122] By forming similar ridges along the top of each of the orthogonalcross-members, and arranging that the ridges of the cross-members areengaged by the wire cloth after the latter have become embedded in theridges of the surrounding flanges, by heating over the whole surfacearea of the frame, so the woven wire meshes will become secured to thecrests of the crossing members which when the assembly cools, will holdthe woven wire cloths firmly in place, and provide additional supporttherefor.

[0123] In a typical example, the first wirecloth will have a mesh sizeof approximately 30 and the second cloth a mesh size in the range100-300. Each is made from annealed stainless steel wire—that making upthe first cloth typically having a diameter of the order of 0.28 mm.

[0124] In accordance with a preferred aspect of the invention, the frameis formed from a plastics material which may be reinforced with glassfibres or similar reinforcing material and the edge regions and thecrossing members of the frame are all reinforced with elongate steelwires which are welded together to form a wire frame reinforcingstructure in a manner which will be described later. Typically steelwire of 2.5 mm diameter is used to construct the reinforcing wire frame.

[0125] A frame such as shown in FIG. 1 is typically formed by insertingthe reinforcing framework into a suitable mould tool and injectingplastics material under pressure into the cool so that the wirereinforcing frame becomes completely encased within plastics material(typically reinforced with glass fibre) so that on release from themould, the reinforcing wires are totally encased within plasticsmaterial.

[0126] The section at CC in FIG. 2 shows the cross member 14 in sectionand the orthogonal cross member 16 is seen extending on either side of14. The edge regions 18, 20 are reinforced by frame members 34 and 36.

[0127] The cross-section at BB in FIG. 1 is shown in FIG. 3. The crossmember 14 is still visible but the orthogonal cross member 38 is nowshown intersecting 14 and the opening intersected by BB is denoted 40.

[0128] Ridges such as 24 in the upper surface of the relatively wideedge flange 20 can be seen, as can also the inner of the two ridgesalong the narrower edge 28, namely ridge 42 which is also identified inFIG. 1 for reference purposes. The ridged end 28 is cut away at oppositeends at 44, 46 as shown in FIG. 1.

[0129] Also shown in FIG. 3 are some of the metal reinforcing rodsforming the reinforcing framework. Two rods designated 48 and 50 extendthrough the upper and lower regions respectively of the cross member 38.

[0130] Similar pairs of rods extend through the upper and lower regionsof each of the parallel cross members such as 16.

[0131] Through each of the orthogonally extending cross membersdesignated 14 and 52 respectively, extend two similar pairs of rodsdesignated 54 and 56, and 58 and 60 respectively. At the points ofintersection between the rods they are welded together as by electricresistance welding.

[0132] Further reinforcing rods 62 and 64 extend parallel to rods 54 and58 through the edge region 20, and this is resistance welded to theextended ends of the rods 48 etc where they extend into the flange 20.

[0133] The extensions of the lower rods such as 50 are bent upwardly asshown at 66 so as to extend co-extensively with or in contact with theunderside of the upper rod of the pair eg extension of rod 48 and areresistance welded thereto. In this way the upper layer of intersectingand welded rods such as 48, 54, 58, 62, 64 is secured to the lower layerof rods such as 50, 56, 60, to form a unitary reinforcing structure.

[0134]FIG. 4 is a part section on AA to a larger scale than FIG. 1 whichallows the ridges 30 and 32 to be seen in section at the ends of theframe which are to be butt-joined.

[0135]FIG. 4 also shows detail of the male and female lockingarrangement at opposite ends of the frame. Thus at the left hand end anelongate protrusion 68 stands off from the left hand end face 70 andextends across the entire width of the frame, whilst at the other end ofthe frame the end face 72 is cutaway to define a rebate 74 to receivethe protrusion 68 of an abutting frame.

[0136] The end face 72 also includes an upstand 76 which cooperates withthe opposite end region of the end face 70 of the abutting frame, tospace the two end faces from each other by the thickness of the upstand76. This also assists in seating the rebate 78 in the protrusion 68correctly below the lp 80 along the upper edge of the rebate 74.

[0137]FIG. 4 also shows the small openings which can be left above theintersection point of the reinforcing grid if stand-off pits, or stools,are used in the mould tooling to position the reinforcing grid withinthe tooling. Two of these are visible at 82 and 84.

[0138]FIG. 5 is a part end view in the direction of arrow D in FIG. 1and shows the upstand 76 and the end face 72 of the right hand end ofthe frame as depicted in FIG. 4. Likewise the lip 80 is also showntogether with the undercut 86 in the underside of the shoulder createdby the rebate 74.

[0139]FIG. 6 shows how two frames 10 and 10′ each similar to that shownin FIG. 1 can be joined, with the two edges 26′, 28 abutting/engaged aswill be described more fully with reference to later figures of thedrawings.

[0140] In use the wider flanges 18 and 20, 18′ and 20′ and one endflange 28′ are gripped in an appropriate manner within the basket of ashaker assembly. The end Flange 20 may in fact only be supported on itsunderside in order to permit solids material to progress over that edgeunimpeded during shaking.

[0141] An inflatable grip seal may be used along the edges 20, 20′, 28′,18′ and 18.

[0142] As shown in FIG. 7, the two frames 10 and 10′ may be aligned andin some shakers advantageously the aligned frames are mounted so as tobe inclined to the horizontal so as to describe an uphill slope up whichthe mud which is to be filtered climbs as a result of the shakingmovement. Typically the shaking movement has components generally in thedirection of the alignment of the two frames and generally perpendicularthereto.

[0143] The two separate parts forming the abutment are shown to anenlarged scale in FIGS. 8 and 9, and detail of the engagement is shownin FIG. 10. FIG. 9 shows the edge 28 which includes the protruding nosegenerally designated 68 which extends across the width of the frame andfits below the shoulder 80, of the rebate 74 of the cooperating frame10.

[0144] The upstand 76 may comprise a separate member in the form of aseal which may be of plastics material or rubber or a composite thereof.Alternatively as shown it may simply comprise a protruding ridge formedduring moulding, in the material from which the rest of the frame isconstructed.

[0145] When mounting the two frames in a shaker, frame 10′ is firstintroduced into the shaker basket and thereafter frame 10. By raisingthe right hand end of frame 10 (as viewed in FIG. 6) so that the frame10 is angled relative to the plane of frame 10′, the nose 68 can beinserted below the shoulder 80 into the undercut 86 of the rebate 74 sothat the ridge 88 of the protrusion 68 engages in the undercut 86 behindthe shoulder 80. Once 88 is within 86, the frame 10 can be lowered so asto be in alignment with 10′ and by choosing the dimensions carefully, inthat condition only, the inclined surface 90 engages the inclinedunderside 92 and the upstand 76′ makes contact with the face 70 of theother frame end. This is more clearly shown in FIG. 10.

[0146] By dimensioning thus any fluid trapped between the ends of theframe as the two frames are pushed together, can be squeezed through thegap left between the surfaces to facilitate drainage through thelowermost gap visible in FIG. 10.

[0147] In some shakers, it is preferred that the two screens such asshown in FIG. 6, (each formed by a frame wash woven meshes stretchedthereover) are inclined relative one to the other instead of beingaligned as shown in FIG. 7. To this end an alternative edgeconfiguration may be incorporated, by arranging that one frame end canbe fitted into the other with a small step between the two uppersurfaces.

[0148]FIG. 11 is a plan view of such a frame.

[0149]FIGS. 12, 13 and 14 provide edge and section views to an enlargedscale of the case of FIG. 14, of the frame shown in FIG. 11.

[0150] Essentially the construction of the central area of the frame issimilar to that of the frame of FIG. 1. Thus the meshes are secured toexternal flanges 100, 102, 104 and 106 and to the upper edges ofintersecting orthogonal cross members such as 108, 110, 112 etc.

[0151] The relatively wide flanges 100 and 102 are visible in FIGS. 12to 14, together with the cross members 110, 112.

[0152] Each of the intersecting cross members is ridged along its upperedge as best seen from the hidden detail of items 110, 112 in FIG. 14.The ridges are identified by reference numerals 114, 116 and it will benoted that the peaks of the ridges 114, 116 are just below the peak ofthe ridges 118 along edge 106 (see FIG. 11). This ensures that when theGRP material is heated to accommodate the wire meshes, the heating andwelding of the meshes to the outer regions 100 to 106 of the frameoccurs prior to that over the central region made up of intersectingcross members 108, 110 etc.

[0153] The ends 104, 106 are designed to be capable of interengagementand accordingly the end 106 is formed with a pair of jaws 120, 122 whichextend across the entire width of the frame end and can receive theopposed end 104 of a second frame (not shown) This opposed end comprisesan extended flange 124 having three ridges 126 formed in its uppersurface, the height of the flanges combined with the thickness of theflange 124 being equal to the internal spacing between the upper andlower jaws 120, 122, so that an end 124 can be pushed into the elongateslot formed by a pair of jaws 120, 122.

[0154] At 128 the underside of the upper jaw diverges upwardly away fromthe lower jaw, to assist in entering the end 124 therein. Being formedfrom a plastics based material, the ridges 126 are deformable, and bydimensioning the gap between the jaws 120, 122 accurately, so the flange104 can become firmly wedged in the gap between those jaws, the ridges126 forming a partial seal between the flange 124 and the underside 128of the upper jaw 120 of end flange 106.

[0155] As with the frame illustrated in FIGS. 1 to 10, the frame of FIG.11 includes a grid of reinforcing rods, welded or otherwise joined atthe interstices of the grid, and constructed from upper and lowerlayers—generally designated 130, 132 respectively—see FIG. 15. The endsof the lower layer of rods are bent upwardly as at 134 in FIG. 16 to bewelded to the aligned rods of upper layer in the end flange 104.

[0156] The upper and lower layers of rods terminate without convergingat the other end of the frame as can be seen from the part section ofFIG. 15.

[0157] The two edges 100 and 102 are reinforced in the same way as isend flange 104, and the ends of the rods 136 of the lower layer are bentupwardly as at 138 in FIG. 17, to merge with and be welded or otherwisejoined to the upper rods 140.

[0158] All the rods are wholly contained within the rectilinear array ofintersecting cross members except at the flanged edges of the frame.

[0159] The part section of FIG. 17 is taken on the line CC in FIG. 11and cross members 110 and 112 are denoted in both figures.

[0160]FIG. 18 shows how two frames such as shown in FIGS. 11 to 17 canbe fitted so that one lies at a shallow angle relative to the other. Theone frame is denoted 14.

[0161]FIG. 18A shows a modified interengagement possibility similar tothe engagement of an end such as 126 in an elongate “socket” betweenjaws such as 120, 122—but in which both ends are “female” (ie equivalentto end 106) , and the upper flange of the one can be received andwedged, into the socket of the other and a good seal is maintained.Similar reference numerals are employed to denote items in common asbetween FIG. 18A and earlier and later figures.

[0162] As also shown in FIG. 17, plastics support inserts or stools suchas denoted by 142, 144 may be located in the mould tool so as to engagethe interstices of the intersecting rods, thereby to correctly space theframework within the mould tool when the latter is closed.

[0163] In an alternative spacing arrangement, the spacing andpositioning may be achieved by fingers protruding from the insidesurfaces of the mould tool, and any openings in the plastics mouldingmay be filled by appropriate material after the tool has been opened andthe moulded component has been removed, or may be filled by the plasticsmaterial from which the struts are formed when the frame is subjected toheat and pressure during the manufacturing process.

[0164] Preferred arrangements utilise stand-off supports or stools sincethese can be used not only to centre the reinforcing framework withinthe mould tool, but where the framework has been constructed so as tobow slightly upwardly and downwardly, they will redefine the correctspacing between the upper and lower arrays of rods, when the mouldtooling is closed. A preferred form of such stand-off support isillustrated and described with reference to FIGS. 19 to 23.

[0165] In order to prevent moulding pressure within the tooling fromforcing the upper and lower walled arrays of rods/wires together so asto reduce the spacing therebetween, spacers 143 (typically bent wires,formed to one or other of the two arrays and permitting relativemovement towards each other beyond a critical dimension defined by thespacers) are provided, as shown in FIG. 17C.

[0166] In a similar way, plastic ferrules such as that shown in FIG. 17and denoted by 146, can be inserted into the mould tool so as to coverthe ends of the rods which might otherwise be left exposed.

[0167] In a preferred mould tool, the welded reinforcing framework issupported in one part of the tooling by four retractable pins (145), twoon opposite sides of the framework, by means of double ended plasticsferrules (146) which are each fitted at one end onto an end of one ofthe rods (140) making up the framework, and at the other end onto one ofthe retractable pins. After moulding, the pins (145) are retracted fromthe blind holes in the ferrules (146) which will have been embedded inthe GRP during moulding, to permit the moulded article to be removedfrom the tooling.

[0168] An example is shown in FIGS. 17A and 17B.

[0169] Where support inserts or ferrules protrude after moulding, theycan be removed by cutting or filing.

[0170] As mentioned in relation to FIGS. 11 et seq, the crests of theridges in the edge flanges of the frame shown in FIGS. 1 to 10 may alsolie in a different plane from the crests or the ridges on the crossmembers such as 14 and 16, and in a preferred arrangement one or more ofthe outer ridges along the C flanges such as 18, 20, 26 and 28 extend(s)above the plane containing the crests of the crests of the cross memberssuch as 14, 16. In this way the peripheral regions of the frames containa greater amount of plastics material to be heated and softened beforethe woven wire mesh cloth can be forced thereinto, than exists along thecross-members. Bonding of the peripheral regions therefore has to occurbefore the cloths can come into contact with, and become bonded into thecrests of the cross members 14, 16 etc.

[0171] FIGS. 19 to 23 show, to different, and generally an enlargedscale, the location of preferred stand off supports for the reinforcingrods. Here each support comprises a tapering chisel-ended metal pin 148as shown in FIG. 19. The chisel end 150 is best seen in FIG. 21.

[0172] As shown in FIG. 23, the pins are arranged so that each chiselend 150 extends generally perpendicularly to the length dimension of therod 48 which it will engage in the mould tool.

[0173] By virtue of the chisel end 150, and the taper 152, liquidplastic material flows around the tapering, chisel ends in such a way asto leave similarly tapering recesses such as 154 in the crests of thecross members of the frame when the tool is opened and the chisel endsare extracted from the moulding. Each recess left by a pin 148 thereforeonly exposes a tiny area 156 of the wire (see FIG. 20).

[0174] When woven wire cloths are stretched over the crests of the frameand the cloths and frame are heated, the plastics material flows intothe lower end of each recess left by a chisel end, such as 150. Thisfills the tiny area to provide a barrier between the wire mesh(es) andthe reinforcing rods such as 48 (see FIG. 3).

[0175]FIG. 24 is a perspective view of the female end of the frame 10′of FIG. 9.

[0176] The sealing ridge 76 can be seen above the groove 74 into whichthe elongate male “nose” 68 (see FIG. 9) fits.

[0177] Across the frame are stretched woven wire cloths, the upper oneof which is shown in part at 158 and the lower, in part, at 159. Twosuch cloths, one above the other, are stretched over the frame, the meshof the lower cloth being coarser than that of the upper cloth, and withthe tension in the lower cloth being greater than that in the upper.

[0178] By heating at least the upper surface of the frame, and applyinguniform downward pressure on the cloths, the crests of the plasticsframe will soften and allow the wire cloths to sink into the crests,which will also become flattened into the process. On cooling theplastics material hardens and secures the wire cloths in place.

[0179] The crests of the outermost ridges 160 on the flange extendupwardly above the crests of the inner ridges of the same flange.

[0180] The land area at the junction between two screens such as shownin FIGS. 7 and 18 should be as small as possible so as not to impede theprogress of particulate material from one screen to the next.

[0181] Likewise the front edge of the screen where solids traverse overand off the edge, for example into a skip, should also be as small inarea as possible.

[0182] The joint provided by interfitting ends such as shown in FIGS.18, 18A, causes the edge 120 of the upper frame to overlie wire mesh ofthe lower frame. The non-screening dead area is thereby reduced to theminimum, ie the ridged end 120.

[0183] The joint provided by the engagement such as shown in FIGS. 18,18A also seals the joint and prevents fluid leakage, and has been foundto reduce the risk of bounce between one frame and the other across thejoint. This appears to be achieved due to the formation of a structuralmember by the reinforcing wires and where these are tensioned, as is thecase when the framework is pushed from above and below in the mouldtool, the framework and plastics material will significantly resistbending and therefore minimise any tendency to bounce.

[0184] Flash heating is preferably used to bond the wire cloths to thecrests.

[0185] Weld wires are conveniently employed between overlying parallelrods such as 48, 66 (see FIG. 3) as designated by reference numeral 49.

[0186] The drawings and related description of UK Patent Specifications2237521 and 2299771 are referred to as illustrating filtering machinesof the type in which screens as described herein may be mounted.

1. A method of constructing a polymer support frame over which wovenwire cloth is to be stretched and secured to form a sifting screencomprising the steps of locating in a mould tool a wire frame assemblycomprising two parallel spaced apart arrays of reinforcing wires,closing the tool, injecting liquid polymer so as to wholly encapsulatethe wire frame and to form an article having an open central regioncriss-crossed by intersecting orthogonal ribs bounded on all sides by arigid flange, in which each of the ribs includes two parallel spacedapart wires of the said wire frame assembly, permitting the polymer tocure, and opening the tool, and removing the moulded article.
 2. Amethod as claimed in claim 1, wherein the wire frame is selected so asto impart sufficient structural rigidity to the support frame as toprevent deflection thereof and consequent changes in the tension in thewire cloth when fitted thereto.
 3. A method as claimed in claim 1,wherein the wire frame is selected so as to impart sufficient strengthto the support frame as to allow the latter to withstand shear stressesintroduced as the frame is clamped into a vibratory screening machine.4. A method as claimed in claim 1, wherein the wire frame is formed fromhigh tensile straightened steel wire, bent as required.
 5. A method asclaimed in claim 4, wherein the wire is of 2.5 mm diameter.
 6. A methodof making a reinforcing wire frame for use in the method of claim 1comprising the steps of: equally spacing apart cut lengths of wire in ajig to form a first array, locating thereover a second equally spacedarray of cut lengths of wire at right angles to the first array,resistance welding the wires of the two arrays at all the points ofintersection so as to form a first rectilinear matrix similarlypositioning two similar arrays of similarly cut lengths of wire in a jigand resistance welding the points of intersection of the orthogonalwires so as to form a second, similar rectilinear matrix, bending in apress break the protruding lengths of wire on at least two of the foursides of one of the rectilinear matrices, so as to bend each protrudingsection first in a generally upward sense and then at a point nearer toits end in a downward sense so that the end region of each protrudinglength extends parallel to the plane containing the matrix, but isdisplaced therefrom, and and thereafter resistance welding the displacedends of the protruding wires of the said one matrix to the protrudingends of the wires of the other matrix.
 7. A method as claimed in claim6, wherein the welding is in part effected through the intermediary oftransversely extending filler wires, so that intersections are providedwhere welds are to be formed between parallel protruding ends of thereinforcing wires, and the filler wares facilitate the resistancewelding of the parallel protruding ends.
 8. A method as claimed in claim6, wherein the wire frame fabrication is assembled so that each matrixis bowed in an outward sense, opposite to the other.
 9. A method asclaimed in claim 6, 7 or 8, wherein at least one spacer is locatedwithin the wire frame fabrication and is attached to one or other of thematrices so as to extend towards the other, whereby any tendency for thematrices to collapse inwards during moulding, is resisted by the spacer.10. A method as claimed in claim 8, wherein the spacer comprises alength of wire bent to form a shallow U with its two ends bent outwardsto form two in-line lugs by which it can be welded to the underside ofone of the wires which form one of the matrices, with the crest of the Usection in close proximity to one of the wires of the other matrix,whereby the spacer will maintain a given dimension between the twomatrices if the fabrication is subjected to a collapsing force duringmoulding, so causing the crest to engage the said wire of the othermatrix.
 11. A method as claimed in any of claims 1 to 5 using a wireframe as claimed in any of claims 6 to 10, wherein an inward force isexerted on opposite faces of the fabrication within the mould tool byfingers protruding inwardly from the inside faces of the tool, toexternally engage the opposite matrices of the fabrication when thetooling closes.
 12. A method as claimed in claim 11, wherein the fingerssandwich the fabrication in position and produce just the requiredinward movement of the two oppositely bowed matrices to render themparallel and spaced apart by the desired distance.
 13. A method asclaimed in claim 11 or 12, wherein the fingers comprise inwardlyprojecting pegs which align with crossing points of wires in the upperand lower reinforcing matrices, to space the matrices from thecorresponding upper and lower internal surfaces of the mould tool andensure that the matrices are buried within the plastics material whichis injected into the mould tool during the manufacturing process.
 14. Amethod as claimed in claim 13, wherein the ends of the pegs taper to anedge, or a point.
 15. A method as claimed in claim 13 or 14, whereinafter the mould tool is opened and the protruding pegs disengage fromthe struts, openings are left in the polymer and the method furthercomprises plugging the openings with plastics material or filler.
 16. Amethod as claimed in claim 11 or 12, wherein the wire frame fabricationis supported within the tooling by means of retractable pins whichprotrude through the tooling wall to engage the fabrication andaccurately locate it within the tooling.
 17. A method as claimed inclaim 16, wherein the pins are retracted as the tooling opens after themoulding step has been completed.
 18. A method as claimed in claim 16 or17, wherein the pins align with protruding ends of wires making up thefabrication. and are separably joined to the ends of the wires by meansof sleeves of plastics material opposite ends of which receive the pinsand the reinforcing wire ends respectively.
 19. A method as claimed inclaim 18, wherein the passage through each sleeve is blocked so as toform two coaxial blind bores, and each sleeve becomes embedded in thepolymer during moulding and remains in the polymer as the pin whichengages it is retracted as the tooling is opened, the blocked passageserving to encapsulate the end of the wire end located in the inner endof the sleeve.
 20. A method as claimed in claim 11 or 12, wherein priorto moulding the tool is fitted with pegs formed from a plastics materialwhich is compatible with or is the same as the polymer material which isto be injected into the mould to encapsulate the wire frame fabrication,and the pegs become integrally bonded therein during moulding so thatwhen the tool is opened, the pegs separate from the tool, and remain inthe frame.
 21. A method as claimed in claim 20, wherein the protrudingportion of each peg is removed by grinding or filing or cutting.
 22. Asupport frame of polymer material having wire reinforcing therein and towhich woven wirecloth is to be bonded to form a filtering screen, inwhich the frame is constructed in accordance with the method of any ofclaims 1 to 5 or 11 to
 21. 23. A filtering screen comprising a supportframe of polymer material with wire reinforcing therein constructed asclaimed in claim 22, and having at least one woven wirecloth tensionedthereover and bonded to one surface thereof.
 24. A filtering screen asclaimed in claim 23, wherein the wirecloth which is first fitted overthe support frame has a coarser mesh than any subsequent layer ofwirecloth fitted thereover.
 25. A filtering screen as claimed in claim23, having two layers of woven wirecloth fitted thereover and bondedthereto, in which the lower wirecloth has a coarser mesh than the upperwirecloth and in which the tension in the wires forming the upperwirecloth is less than the tension in the wires forming the lowerwirecloth.
 26. A wire frame reinforcing fabrication for a support Frameas claimed in claim 22 formed from resistance welded steel wire matricesarranged in two parallel spaced apart planes and themselves weldedtogether along at least two edge regions by welds between protrudingends of the wires of the two matrices.
 27. A method of forming a wireframe reinforcing cage for incorporating in a mould tool for moulding apolymer material therearound to form a support frame for a filterscreen, wherein the cage is constructed from two similar rectilineararrays of resistance welded wires and wherein the method involvesbending protruding wires along at least two sides of one of the arraysand welding the ends of the bent portions of the wires of the one arrayto the protruding ends of the wires in the other array, so as tomaintain separation between the two matrices.
 28. A method ofconstructing a filter screen comprising the steps of forming a polymersupport frame having therein a plurality of similarly sized rectilinearapertures defined by an integral rectilinear matrix of wire reinforcedstruts of polymer material, in which the upper edge of each strut, andthe upper surface of each boundary of the support frame, is ridged, andin which the woven wirecloths are fitted over the ridges, tensioned, andsecured in place by heating at least the ridges so as to soften thepolymer material therein sufficiently to allow the wirecloths topenetrate the crests thereof and upon cooling to remain embeddedtherein, so as to maintain tension in the wires of the wirecloths aftercooling.
 29. A method as claimed in claim 28, which also involves thestep of differentially tensioning the wires in one wirecloth relative tothose in the other, so that different tensions exist in the wires of thetwo cloths after bonding to the polymer support frame.
 30. A method asclaimed in claim 29, wherein the reinforcing matrix is formed from hightensile steel wire of nominally 2.5 mm diameter, grass reinforced gasblown polypropylene polymer is injected into the mould tool underpressure and is left to cure for a given period of time, and the mouldedsupport frame then removed, a first woven wirecloth of nominally 30 meshformed from stainless steel wire of nominally 0.28 mm diameter is fittedover the support frame and tensioned, a second woven wirecloth is fittedover the first wirecloth, the second cloth being formed from wire havinga smaller diameter and a finer mesh size than the first cloth, and islikewise tensioned, a force is applied over the face of the supportframe carrying the overlaid wirecloths, heat is applied to soften thecrests of the ridges and to allow the two cloths to become embedded inthe crests of the ridges such that after the heating and force isremoved and the frame has cooled, the cloths remain bonded to the crestsof the ridges and residual tensions exist in the wires forming the twocloths.
 31. In a support frame which is to have woven wirecloth bondedto the one face thereof by locally heating the frame material andforcing the wirecloth wires into the softened material before it isallowed to cool and harden again, and which is formed from glassreinforced plastics material having embedded therein a reinforcing wireframe constructed as claimed in any of claims 6 to 10 or 27, thethickness of the plastics material between the reinforcing wires and theface of the support frame members to which the woven wirecloth is to bebonded is selected so as to be sufficient to enable the wirecloth to beembedded therein without making contact with the reinforcing wires. 32.in a support frame as claimed in claim 31, the plastics materialcomprises a polypropylene or a polyethylene.
 33. In a support frame asclaimed in claim 32, the plastics material is gas blown and glass fibrereinforced.
 34. in a support frame which is formed from glass reinforcedplastics material, the faces of the peripheral edge regions of thesupport frame which woven wirecloth is to overlay and to which the clothis to be bonded by heating, are formed with a plurality of closelyspaced apart parallel ridges so that when the surface is heated, thecrests of the ridges soften, and woven wirecloth laid thereover andtensioned can, under an appropriate downward loading, penetrate andbecome embedded in the softened crests.
 35. A method of constructing afilter screen using a support frame as claimed in claim 34, wherein theheat is applied through the woven wirecloth so as to preferentially heatthe crests of the ridges as opposed to the remainder of the supportframe.
 36. In a support frame as claimed in claim 34, which is generallyrectangular, the ridges along each of the four sides run parallel to thelength dimension of each side so that the ridges in the surfaces of thefour edges of the frame run perpendicular to the direction in whichwoven wirecloth is tensioned relative to those edges.
 37. A supportframe as claimed in claim 34 or 36, wherein the frame includes a matrixof wire reinforced struts defining within the boundary of the frame aplurality of openings, and sufficient plastics material exists in eachof the struts between the internal wire reinforcement therein and thecloth engaging surface thereof, to enable the strands of wire formingthe wirecloth also to become embedded in the struts when the frame isheated and an appropriate force is applied, without the wirecloth makingcontact with the internal reinforcing wires.
 38. In a support frame ofglass fibre reinforced plastics material over which wirecloth is to bestretched and bonded thereto to form a sifting screen, and whichincludes a matrix of struts within a perimeter flange of the frame, thethickness of the cloth engaging ends of the struts is less than that ofthe more remote regions of the struts.
 39. In a support frame as claimedin claim 37 or 38, the wirecloth engaging edge of each strut is ofreduced section, and on heating the wirecloth wires become embedded inthe said reduced strut sections.
 40. In a support frame as claimed inclaim 39, the reduced section of each strut is created by forming aridge along the wire receiving edge thereof.
 41. In a support frameformed from glass fibre reinforced plastics material over whichwirecloth is to be stretched for bonding thereto to form a siftingscreen and which includes an outer peripheral flange surrounding acentral region occupied by an integral matrix of interconnecting struts,the flange and struts are ridged where they are to engage the wireclothand the ridges extend to different heights so that the crests lie indifferent parallel planes.
 42. In a support frame as claimed in claim41, the crests of the ridges of the struts defining the integral matrixlie in a first plane, which is below a second plane containing thecrests of the ridges on the surrounding flange.
 43. In a method ofcovering a support frame as claimed in claim 42, heat is applieduniformly over the entire face of the support frame by means of by aflat heated platen which forces the wirecloth into the crests of theridges and the wirecloth has to be pressed into the crests of the ridgeson the surrounding flange before it can enter the crests of the ridgeson the struts making up the integral matrix.
 44. In a support frame asclaimed in claim 41 or 42, outer ridges on some or all of thesurrounding flange extend to a greater height than inner ridges on theflange, so that the crests or the outer ridges occupy a third planeabove the second plane, whereby there is a greater volume of polymer tobe melted and spread by the application of heat and pressure to thewirecloth near the outer edges of the peripheral flange, than is thecase near the inner regions of he flange.
 45. A frame as claimed in anyof claims 41, 42 and 44, wherein the overall shape is a rectangle andthe flange which extends along the two longer sides is wider than theflange along the two shorter sides, and at least one of the shortersides is adapted to be joined to the corresponding shorter side of asimilar screen support frame, constructed in the same way as the first.46. A sighting screen constructed from a support frame as claimed in anyof claims 41 to 45, with wirecloth stretched thereon and hermeticallybonded thereto.
 47. Two screens as claimed in claim 46 insofar as it isdependent on claim 45, when mounted in a supporting framework within avibratory screening machine, with the shorter edges of the screensupport frames joined so that the two screens form a larger area forfiltering.
 48. A vibratory screening machine when fitted with twoscreens as claimed in claim 47, wherein one of the screens is mounted soas to be substantially horizontal in use and the other is mounteddownstream of the first and is inclined upwardly in the direction ofmovement of particulate material through the machine.
 49. A supportframe as claimed in claim 45, wherein one of the shorter ends of theframe is adapted to interlock and sealingly engage with a correspondingedge of an adjoining similar support frame such that if the two framesare slid the one towards the other, engagement and sealing occurs merelyby the sliding movement of the one frame relative to the other.
 50. Asupport frame as claimed in claimed 49, wherein the sealing engagementof one frame to the other occurs when the two frames are in line.
 51. Asupport frame as claimed in claim 49, wherein the sealing engagementoccurs when the two frames are mutually angled.
 52. A support frame asclaimed in claim 39, wherein the cross section of each strut isgenerally rectangular and the longer dimension is generallyperpendicular to the plane of the frame, when in a first reinforcingwire extends through each strut near the edge thereof which is to bebonded to wirecloth, and a second parallel reinforcing wire extendsthrough each strut near to the opposite edge thereof, and the wires arebonded to the plastics material by the moulding step and form therewitha beam, thereby to impart rigidity to the structure.
 53. A support frameas claimed in claim 39, wherein the struts intersect similar strutswhich extend at right angles and a second assembly of parallelreinforcing wires is provided, running perpendicular to the firstassembly of wires in planes proximate those containing the first saidassembly, so that a pair of parallel spaced apart wires extends througheach of the struts.
 54. A support frame as claimed in claim 53, whereineach reinforcing wire which aligns with the peripheral flange of theframe extends into the flange at each end thereof, thereby to increasethe rigidity of the flange.
 55. A support frame as claimed in claim 54,wherein the ends of the other wire of each pair are bent so as also tobecome aligned with the flange of the frame, and the bent ends thereofextend into the said flanges close to the ends of the first mentionedwire to further assist in reinforcing the said flanges.
 56. A supportframe as claimed in any of claims 53 to 55, wherein the wires touch atall intersections and are welded at all points of intersection.
 57. Asupport frame as claimed in claim 55 or 56, wherein the ends of eachpair of wires are welded where they occupy the flange.
 58. A supportframe as claimed in claim 56 or 57, wherein cross-point engagement isintroduced between parallel wires in the flange by incorporatingintermediate transversely extending filler wire, or weld wire, betweenthe wire ends.
 59. A support frame as claimed in any of claims 56 to 58,wherein further reinforcing is provided in the flange by means ofadditional reinforcing wires extending parallel to the length directionof the flange so as to overlie or underlie the protruding reinforcingwires entering the flange from the struts, and the additionalreinforcing wires are welded to the protruding reinforcing wire ends.60. A filter screen comprising a support frame as claimed in any ofclaims 22, 31-34, 36-42, 44-45 or 49-59 having wirecloth bonded thereto,wherein the wires of the cloths are taut and under tension in thefinished product and to this end the wirecloths are put in tension atleast while the frame material cools and sets hard, to band thewirecloths to the frame.
 61. A method of repairing or refurbishing afilter screen constructed in accordance with any or claims 23-25, 46 or60, wherein worn or damaged wirecloth is stripped from the surface ofthe polymer frame, fresh cloth is placed over the frame and tensioned asappropriate and heat is applied so as to soften the surfaces of theframe over which wirecloth is stretched, so that the latter canpenetrate the softened plastics material and become embedded therein,after which the assembly is allowed to cool, the tensioning force isremoved, and the wirecloth edges are trimmed back to the surroundingflange of the frame.
 62. A method of repair or refurbishment as claimedin the preceding claim, wherein plastics material is applied to thesurface of the frame which is to receive the wirecloth before the latteris fitted thereover, to provide additional plastics material for bondingthe wirecloth to the frame.
 63. A method of repairing or refurbishing afilter screen as claimed in claim 61 or 62, wherein the stripped supportframe is inserted in a mould and fresh polymer material is injected intothe mould so as to reform on the surfaces of the frame ridges similar tothose which existed when the frame was first manufactured, before thewirecloth is applied thereto.
 64. A method of repair or refurbishment,wherein after removing worn or damaged wirecloth from a support frame,plastics sheet similar in size and pattern of openings to the supportframe when viewed in plan is placed over the frame which is to berepaired, in alignment therewith, before or after new wirecloth isstretched thereover and before heat and pressure is applied, to provideadditional plastics material to bond the new wirecloth to the frame. 65.A repaired or refurbished filter screen wherein worn or damagedwirecloth has been replaced by fresh wirecloth in accordance with themethod of any of claims 61-64.
 66. Apparatus for repairing orrefurbishing a filter screen from which worn wirecloth has been strippedcomprising a tray into which the stripped frame is inserted, wireclothstretching means surrounding the tray including attachment means forsecuring to the edges of a sheet of wirecloth laid over the uppersurface of the frame in the tray, tensioning means for exerting tensionon the wirecloth in at least two different directions whilst it is sostretched over the frame, means for heating the frame to soften theuppermost edges of the matrix of struts and periphery of the frame, andmeans for forcing the wirecloth into the softened material such thatafter cooling, the wirecloth remains bonded to the polymer frame, andcan be trimmed back to the edges of the frame.
 67. Apparatus as claimedin claim 66, comprising further attachment means and tensioning means toenable at least a second sheet of wirecloth to be secured over the firstfor bonding to the frame.
 68. Apparatus for refurbishing a support frameas claimed in either of claims 66 or 67, wherein additional means isprovided for applying plastics material along each of the surfaces towhich the wirecloth is to he attached prior to or after wirecloth hasbeen stretched over the support frame, so as to increase the amount ofplastics material available to bond the wirecloth to the frame.
 69. Asupport frame for a filter screen constructed and arranged substantiallyas herein described with reference to and as illustrated with referenceto the accompanying drawings.
 70. A method of making a support frame fora filter screen substantially as herein described with reference to andas illustrated in the accompanying drawings.
 71. A filter screenconstricted, arranged and adapted to operate substantially as hereindescribed with reference to and as illustrated in the accompanyingdrawings.